A good map can be as gripping as a good novel - none more so than the A-Z of London, whose pages are so rich that I find it hard to put down, often missing my stop because I'm lost in the evocative names and elaborate patterns of the city streets. Sarah Hartley, who span this biography of the atlas's creator out of a magazine article, clearly shares this love of London's names, and scatters them about liberally: Silk Mills Path, Carpenters Mews, Ironmonger Row, Saddlers Mews, Slippers Place, Smithy Street. It's a roll-call of London's history which no cabbie can do without.

Phyllis Pearsall ('Mrs P') created the A-Z out of frustration, ambition, and a sense of lost bearings. The child of a Hungarian-Jewish father and an Irish-Italian mother, her place in the English class system was not entirely clear - and was further confused by four years at Roedean, during which her parents would turn up separately and unannounced to make noisy scenes in the dining hall, her father calling her mother a whore, she denouncing him as a philanderer.

Almost in revenge on this oversexed couple, Phyllis entered into an ascetic marriage that was never consummated. She eventually left her husband, and made a living painting portraits. Constantly getting lost, and losing commissions because she couldn't find her subjects' homes, she imagined a new kind of atlas, which would mark every street including house numbers. With obsessive determination, she set off in 1936 to walk the streets of London, all 23,000 of them, mapping and annotating as she went. After a year of this she had her map. Her first order was from WH Smith, and was delivered in a borrowed wheelbarrow.

The history of the Geographers' Map Company, which Phyllis set up to run the A-Z series, is of limited interest. But this book is held together by the character of Phyllis herself - single-minded but exotic. Sarah Hartley's style is on the light side, and this reads like a magazine article that just kept rolling. But it's a fascinating story, and a marvellous testament to the romance of maps and mapmaking.